Holographic gun sights are well known, but typical designs are complex and may be bulky or have high energy usage. Adjusting a holographic gun sight for windage and elevation has also presented challenges. Adjustment is required to align the positioning of the reconstructed reticle and to compensate for various weapon types and targeting procedures. Existing systems have drawbacks. Accordingly there exists a need in the art to provide alternative or improved designs for holographic gun sights.
Specifically, laser diodes are used in a wide variety of applications that require a narrow spectral width. However, the wavelengths of the light produced by the laser diode vary depending on a number of factors, including the temperature of the laser diode. For example, some laser diodes will exhibit a shift in output wavelength of approximately 0.30 nm/° C. The change in temperature of the laser diode may be due to environmental conditions or due to heating from operation of the diode. For some applications, this shift in wavelength is not a problem. However, for other applications, such as the holographic gun sight, this shift in wavelength will cause the holographic gun sight to be inaccurate.
In a holographic gun sight the hologram reconstructs an image of a reticle which will appear in focus at a distance in the viewing field (Virtual Image Plane). The sight is designed so that this image will overlap the target. Holographic diffractive optics are wavelength dependent, thus very sensitive to changes in laser diode wavelength changes. As the wavelength of the light shifts, the diffraction angle from a holographic element will change, which will result in movement of the projected holographic image and give an inaccurate reticle position relative to the target.
To correct for this change in wavelength some sights are configured such that the system of holographic elements are achromatic and compensate for changes in wavelength. However, it remains desirable (simpler design, easier manufacturing, more reliable, lower cost) to provide a source of laser-light in which the output wavelength is stable as the temperature changes within an operating range. Similar considerations apply to other devices utilizing laser light such as stable LED, RCLED . . . etc.
One approach to addressing this problem is to control the temperature of the laser diode, such as through the use of a thermoelectric device or TEC cooler. Such control may be open or closed loop. An open loop control may be used, such as a temperature sensor attached to the laser diode. As the sensor temperature changes the TEC cooler will be adjusted to keep the diode at one stable temperature. For a closed loop system the wavelength output by the laser diode may be directly monitored by a device such as a grating. This information is then used to adjust the temperature of the laser diode via the thermoelectric cooler, and bring the diode back to the target wavelength. While thermal control of the laser diode is effective in preventing a change in wavelength, thermoelectric controllers are large in comparison to the laser diode and may draw a current in excess of 0.5 amps. For either case using a thermoelectric cooler increases the physical size of the laser source assembly and greatly increases its energy requirements. For this reason thermoelectric controllers in gun sights are impractical and undesirable at this time. By way of example, US Patent Application Publication No. 2014/0064315 discloses another means for incorporating a thermoelectric controller in a gun sight.
An alternative type of semiconductor laser diode is known as a VCSEL, or vertical-cavity surface-emitting laser. A VCSEL has improved temperature stability as compared to a standard laser diode. For example, a VCSEL may have a wavelength shift of approximately 0.05 nm/° C., which is approximately a six-fold improvement over a standard laser diode. While this is a large improvement over the standard laser diode, even this smaller amount of wavelength shift will be enough to impair the accuracy of the device.
Parallax mismatch is an undesired result of using holography for target shooting. Parallax mismatch results when the reconstructed reticle, also referred to as a perceived image, is displayed on an object either closer or further than the intended distance. This means that the reconstructed reticle will move around as the viewing eye moves. If the reconstructed reticle is displayed on an object at the predetermined distance from the hologram apparatus, then the image should not move. It is desired that the reconstructed reticle remains still as the viewing eye moves about a display hologram when the reconstructed reticle is displayed on objects at varying distances. This effect would improve shooting accuracy and precision. Accordingly, there is a need for a holographic weapon sight that corrects for parallax mismatch.
A dual H.O.E. assembly can present several challenges. For example, if the H.O.E. alignment in use does not match near perfectly with how the H.O.E. was recorded, then undesired imaging results. Accordingly, a need exists to provide weapon sights with improved holographic imaging in a compact and convenient setting.